MAINTAINERS: add YAML file for i2c-demux-pinctrl

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Author: Wolfram Sang

MAINTAINERS

@@ -8711,6 +8711,7 @@ F:	include/linux/phy/
 GENERIC PINCTRL I2C DEMULTIPLEXER DRIVER
 M:	Wolfram Sang <wsa+renesas@sang-engineering.com>
 S:	Supported
+F:	Documentation/devicetree/bindings/i2c/i2c-demux-pinctrl.yaml
 F:	drivers/i2c/muxes/i2c-demux-pinctrl.c
 
 GENERIC PM DOMAINS